Abscisic acid (ABA) is a natural occurring hormone found in all higher plants (Cutler and Krochko 1999, Trends in Plant Science, 4:472-478; Finkelstein and Rock 2002, The Arabidopsis Book, ASPB, Monona, Md., 1-52). ABA is involved in many major processes during plant growth and development including dormancy, germination, bud break, flowering, fruit set, general growth and development, stress tolerance, ripening, maturation, organ abscission, and senescence. ABA also plays an important role in plant tolerance to environmental stresses, such as drought, cold, and excessive salinity.
One key role of ABA in regulating physiological responses of plants is to act as a signal of reduced water availability to reduce water loss, inhibit growth and induce adaptive responses. All these functions are related to stomatal closure (Raschke and Hedrich 1985, Planta, 163: 105-118). When stomata close, plants conserve water to survive environmental stresses. However, stomatal closure also can result in reduced photosynthesis, respiration and growth. Stomatal closure is a rapid response of plants to ABA. The mechanism of this effect has been studied and has been shown to be due primarily to ABA effects on guard cell ion channels. Specifically, ABA blocks H+ extrusion and K+ influx from guard cells and promotes K+, Cl−, and malate extrusion and Ca2+ influx. The net effect of ABA is to reduce the total osmotica in the guard cells, which in turn decreases the water content in the cell. This causes the guard cells to lose turgor and thus close the stomata (Assmann 2004, In: Plant Hormones Biosynthesis, Signal Transduction, Action, ed. Davies, p 391-412). The closing of the stomata results in reduced transpiration. The reduction of transpiration caused by stomatal closure is widely used as an experimental technique to indirectly identify and quantify ABA activity. The ability of ABA to reduce water use can not only extend the display shelf life of ornamentals or the postharvest shelf life of leafy plants, or promote drought tolerance, but it also can lead to a reduction in cold stress injury (Aroca et al. 2003, Plant Sci., 165: 671-679). ABA-induced reduction of stomatal conductance can lead to a decrease in photosynthesis (Downton et al. 1988 New Phytol., 108: 263-266) which in turn can lead to growth control. Improving the performance of ABA may be useful not only for improving the reduction of transpiration and water loss, but also for other uses of foliar applied ABA such as maintaining dormancy of buds and seeds, controlling fruit set, accelerating defoliation, and enhancing color development of fruit such as grapes.
Surfactants or adjuvants have long been used with pesticides and plant growth regulators to increase the absorption or uptake by plants and thus improve the performance of the applied chemicals. Adjuvants include wetter-spreaders, stickers, penetrants, compatibility agents and fertilizers. However, there is little prior art information about adjuvant effects on ABA efficacy. In the patent application of Quaghebeur (2005, US20050198896 A1) it is noted that “ethoxylated sorbitan esters and siloxanes have proved to be particularly suitable for the application of ABA”, but there is no mention of adjuvant effects on ABA efficacy. Lee et al. (1997, Kor. Soc. Hort. Sci. J., 38:717-721) reported that the addition of 0.05% Tween 20 (a commercially available ethoxylated sorbitan ester) improved ABA effect. However, Tween 20 is used for academic research and not packaged and distributed for the agricultural market.
The pH of an exogenously applied ABA solution may play a role in determining the efficiency of ABA uptake by plants. At an acidic pH, ABA is in its neutral undissociated form. This form is more lipophilic, and its penetration of the plant cuticle would be favored relative to the charged, dissociated form of ABA present at higher pHs (Blumenfeld and Bukovac 1972, Planta, 107:261-268). The uncharged undissociated form of ABA would more easily cross cell membranes from the relatively acidic apoplast into the cytosol.
Foliar applied nitrogen fertilizers, such as urea or ammonium nitrate, have been used in combination with plant growth regulators (PGRs) to improve the performance of the PGR. For example, the combination of the PGRs benzyladenine (Naito et al. 1974, J. Japan. Soc. Hort. Sci., 43: 215-223) or gibberellic acid (Shulman et al. 1987, Plant Growth Regul., 5: 229-234) with urea increased the grape berry sizing effect compared to the sizing effect achieved with the PGR alone. Ammonium salts have been reported to increase the absorption of pesticides (Wang and Liu 2007, Pestic. Biochem., Physiol., 87: 1-8). Nooden (1986, U.S. Pat. No. 4,581,057) claims the use of ABA to increase fertilizer performance. However, there are no reports of the use of urea (H2NCONH2) or ammonium nitrate (NH4NO3) to improve ABA performance.
In order maximize the performance of ABA in its various agricultural and horticultural applications; there is a need to improve the extent and duration of ABA efficacy.